1. Field of the Invention
This invention relates to an image forming apparatus such as a copying apparatus or a printer for effecting image formation by an electrophotographic system, and particularly to an image forming apparatus for effecting image density processing by digital processing.
2. Related Background Art
In the image formation by an electrophotographic system, various systems have heretofore been adopted to obtain gradation images. The developing process by an image forming apparatus according to the prior art has the characteristic that low density portions are liable to be blown off and high density portions are liable to be crushed, and usually images formed by the electrophotographic system are liable to become contrasty. Also, the technique of attempting the reproduction of halftone by adjusting the potential contrast which is the potential difference between the image portion and non-image portion of an electrostatic latent image formed on an image bearing member is generally widely known.
Now, in a digital electrophotographic system wherein an image reading portion for introducing input image data and an image forming portion (image writing-in portion) for forming an image conforming to the input image data are made independent of each other and an image signal read by the image reading portion is digital-processed and sent to the image forming portion, there is known a method of making image data while referring to a table for correcting an input-output characteristic, and the preservation of macroscopic image density is possible.
Generally, a density correction table is made as the reverse characteristic of the output characteristic of the image forming portion. That is, stepwise output data are sent to the image forming portion, the density of an outputted image is measured, and the value of image data for obtaining predetermined density is found conversely.
Also, when for a pixel, gradation expression is done by the use of an area gradation method or an intensity gradation method, data for reproducing the density of a predetermined pixel in the read image density data are sent to the image forming portion to thereby effect the reproduction of image density.
Image density is generally measured as the density of a uniform image of a certain size, and usually it is often within a range of several millimeters to several centimeters.
In the prior-art image forming apparatus described above, however, when image density processing is effected by digital processing, the mutual action between pixels is great and the density of a certain marked pixel is changed by the data around that pixel and it becomes impossible to effect good density control. Particularly in recent years, as the image forming apparatus becomes higher in resolution, the density interference between pixels becomes more liable to occur.
Some specific examples of the prior-art image density control will hereinafter be described with reference to FIGS. 25 to 29 of the accompanying drawings.
FIG. 25 shows a bayar pattern of 4.times.4 pixel dithers, and this pattern is directed to smear away the pixels in the order of numbers to thereby effect area gradation control and obtain a linear gradation property. FIG. 26 shows the actually measured image density when this area gradation control is applied to the electrophotographic system. The gradation property in this case becomes actually not linear, and the reversal of density is caused.
FIG. 27 typically represents the surface potential distribution of an image bearing member (photosensitive drum) of dither bayar pattern gradation 4 by a simulation. In such a simulation, there are shown the average potential (a), the lowest potential (b) and the highest potential (c) of the image bearing member (photosensitive drum) at each level (gradation). FIG. 29 shows the result of the measurement of the potential difference between the lowest potential and the highest potential of a pixel at each level (gradation), i.e., the contrast between pixels.
Thus, it will be seen that the reversal of image density is caused by the non-linearity of the contrast between pixels by the interference between the pixels.
Also, such partial reversal of image density becomes remarkable phenomenon by forming a minute electrostatic latent image to improve the property of the image, and using a toner of a small diameter (the average diameter being 8 to 6 .mu.m).
Also, the measurement or electrolytic measurement of the potential distribution of the latent image in a minute area for confirming such an effect is nearly impossible from the measured size and therefore, it was impossible even to obtain data for studies.